1. Field of the Disclosure
The present disclosure relates to signal processors, and more particularly, to signal processing using sigma delta converters.
2. Description of the Related Art
A frequency modulation/demodulation approach is commonly used to transmit digital data over a wireless system. A transmitting modulator within a transmitting device modulates a baseband signal at a carrier frequency into an antenna. A demodulator in a receiving device provides a signal, commonly referred to as a baseband signal b(t), that is representative of the information being sent from the transmitting device to the receiving device. The demodulator extracts (i.e., demodulates) the baseband signal b(t) from the carrier frequency, that is, the high frequency wireless signal that “carries” the baseband signal b(t) through the medium (e.g., airspace) separating the transmitting and receiving devices.
If the baseband signal corresponds to a given logic value, for example, in a digital Frequency Shift Keying modulation scheme (FSK), the baseband signal represents a “1” when the signal has a frequency of (fcarrier+f0). If the baseband signal represents a “0”, the signal has a frequency of (fcarrier−f0). Thus, the signal launched alternates between frequencies of (fcarrier+f0) and (fcarrier−f0) depending on the value of the data being transmitted. Note that in actual practice the transmitted signal can have a profile that is distributed over a range of frequencies in order to prevent that a large, instantaneous changes in channel propagation in a small frequency range distorts significantly the signal. The carrier frequency fcarrier corresponds to the particular wireless channel that the digital information is being transmitted within. In typical wireless communications systems the digital modulation schemes are more complex like in Quadrature Phase Shift Keying (QPSK) and Quadrature Amplitude Modulation (QAM) systems. In some communications systems signal spreading is used by multiplying the baseband signal by higher frequency binary code, as in Wideband Code Division Multiple Access systems (W-CDMA).
The net result of such techniques is that the bandwidth of the modulated RF signal is increased and can have a significant Amplitude Modulated (AM) component. Therefore, more linearity is needed in the receiver circuits to avoid inadvertently demodulating the AM component by a second-order or third order non-linearity in the amplifiers and mixers, resulting in translating the whole signal back to baseband and creating a DC signal that can be several orders of magnitude higher than the signal being detected. Such a shift in DC magnitude makes it harder to detect the transmitted signal and can cause clipping.
In a Direct Conversion Receiver (DCR), there is another cause of DC offsets related to the Local Oscillator (LO) frequency leaking out to the antenna as a result of the LO frequency being the same as the RF channel. In this scenario the LO signal is thereby transmitted and subsequently reflected back to the antenna by obstructions nearby. When this reflected signal reaches the mixer, it is multiplied by the LO signal which converts it down to DC level, where, due to its high DC power component, it can clip the baseband components after the mixer, thereby wiping out the desired signal.
Another leakage path for the LO frequency is through the mixer: from the LO port to the RF port directly. In both cases the effect is a high DC offset component that is destructive of the desired signal, causing non-linearities.
Such a DC offset should be corrected to assure proper interpretation of the transmitted signal. Current DC offset correction schemes put a heavy burden on the noise specifications of the whole receive line-up by injecting noise into the receive path. As CMOS-only radios become popular, that is, as radio frequency (RF) circuitry and baseband circuitry are increasingly combined onto a single CMOS integrated circuit, noise injection reduction becomes increasingly important.
A DC offset correction technique that minimizes the amount of noise injected into the receive path so as to minimally impact the noise figure of the receiver is desired.
The use of the same reference numbers in different drawings indicates similar or identical items.